Microelectromechanical systems (“MEMS”) are used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a structure suspended above a substrate, and associated electronics that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
The associated electronics, substrate, and movable structure typically are formed on one or more dies (referred to herein simply as a “die”) that are secured within a package. For example, the package, which typically encapsulates such a die, may be produced from ceramic or plastic. The package includes interconnects that permit the electronics to transmit the movement data to the external devices. To secure the die to the package interior, the bottom surface of the die commonly is bonded (e.g., with an adhesive or solder) to an internal surface (e.g., a die attach pad) of the package. Other package designs simply cap the MEMS die to provide a so-called “chip-level” package, which can be directly coupled with a printed circuit board.
Problems can arise, however, when the temperatures of the two surfaces change. For example, the temperature of a chip-level package substrate and that of an underlying printed circuit board can change. When both have different coefficients of thermal expansion (“CTE”), the board can apply a mechanical stress to the substrate of the die. The same stress can arise when the aggregate CTE of the package and printed circuit board differ. This stress undesirably can bend, torque or flex the die substrate to an unknown curvature. Substrate bending or flexing consequently can affect movement of the die structures and the functioning of the electronics, thus causing the output data representing the property being measured (e.g., acceleration) to be erroneous.
In a similar manner, mechanically induced linear or torsional stress applied to the circuit board also can be translated to the die, thus causing the same undesirable effects.